Status of SuperKEKB
Accelerators
Makoto Tobiyama（KEK Accelerator Laboratory） for SuperKEKB
Accelerator Team1

Recent operation2

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Especially on 2022b runs
 Strongly required to get higher “peak” luminosity for the 10years evaluation process of MEXT.
• Decided to skip various machine studies to understand machine.
• Could not repair damaged collimator heads.
• Needed to skip the regular maintenance.

 Due to crazily rising electricity costs, we had no choice but todiscontinue operation and enter LS1.
• The rising of electricity costs is continuing– we are doing our best to reduce thestandby power after stopping the operation of 2022b.
• (Almost) stop the water pumps.
• Stop the air conditioning of the power supply buildings and the arc section of thetunnel.
• Tuning off the lights of the tunnel in the night.
• 25Hz operation of Linac.

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Achieved up to now..
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Peak luminosity : 4.65 x 1034 cm-2s-1 (4.71 x 1034 cm-2s-1 w/o Belle II data taking)Integrated luminosity : 424 fb-1 (491 fb-1)
Peak currents : 1.46 A (LER) / 1.14 A (HER), 2346 bunches (2-bucket spacing)
βy*: 1 mm (0.8 mm) << bunch length ~6 mm -> proof of the nano-beam scheme
Crab waist scheme has been applied (80 % in the LER, 40 % in the HER).
•luminosity improvement

 Beam-Beam parameter : 0.035 at 0.7 mA (0.045 at 1.1 mA for small number ofbunches) Bunch-by-bunch FB tuning (gain, noise reduction) in the HER ->luminosity
improvements
 Bunch-by-bunch FB tuning (number of taps) in the LER ->suppress single bunch
blowup, luminosity improvements
 Chromatic X-Y coupling correction with rotatable sextupoles in the LER >luminosity improvements5 Long-term drift of QCS magnetic field (beta-beat) <-reduced by new QCS
initialization procedure Orbit deviation due to IP knob tuning (beta-beat) <- suppressed with QCS corrector(ZHQC2RP)
 Increase of positron charge for the LER injection : 3 nC at the end of e+ beamtransport line
 2-bunch injection for the LER and HER ->improve injection efficiency
 Adjustment of injection orbit in the HER (septum, kicker) ->improve injectionefficiency (not enough) Reduce leakage orbit from injection kickers <- reduced by additional inductance forthe coils

6Operation Statistics

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Machine parameters

8Challenges to improve luminosity Sudden beam loss and damage to the vertical beam collimators• Damaged collimator(s) increases transverse impedance, which makes thethreshold of TMCI much lower.
• Not easy to increase the bunch current.
• Will be presented in the Ikeda-san’s talk.

 Beam blowup in the LER• Some (or most) of the sources might be coming from the higher transverse beamimpedance.

 IR Optics (βy*) modulation due to stored current (HER)• Beam-line deformation, especially around the strong sextuplole magnets.

 Shorter beam lifetime, especially in LER
 Injection efficiency and stability of the injector• Emittance growth coming from beam transport line
• Injection background9

Sudden beam loss
 Without growing the transverse
motion, some part of bunches
drops within 1-2 turns.
• Occurs in both LER and HER, but thedamage in LER is much greater (QCSquench, vertical collimator damage, etc)
• After damaging the collimator heads,
many unwanted side-effects happen.
• Much larger background.
• Larger transverse beam impedance.

 Started ITF-sudden beam loss
subgroup.

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Beam(bunch) current

12Beam blowup in LER
 Bunch current dependent– TMCI related.
• With the increase of the bunch current, the synchro-betatron sideband (νy+νs)
appears (and will finally merge).
• Also coupled with the side-effect of transverse BxB feedback system.
• Large phase shift coming from the many-tap filter (10 tap FIR) reduced the gain
margin of the system, finally excited dipole oscillation at the sideband.
• By reducing the number of FIR taps (10 ->4), the phase shift in the digital filter wasmitigated and the dipole oscillation at the sideband was suppressed.

13 The threshold seems depend on the aperture of verticalcollimators (transverse impedance), selection of vertical tune,vertical chromaticity, BxB FB settings + damage of the head ofvertical collimator.
 With the severe damage of the heads of the vertical collimator,
the transverse impedance has been increased and thethreshold had dropped less than 0.8mA/bunch.
 Non-linear collimator should help to reduce the transverseimpedance.

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IR Optics (βy*) modulation due to stored
current (HER)

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Long Shutdown 118

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Non-linear collimator20

QCS-R cryostat leak hunting
 QCS-R and L have been retreated.
• Leak check on QCS-R cryostat has been made21

HER injection line physical aperture22

(LS1) upgrade for Injector
 Fast pulse magnets will be installed at the J-arc and the sector 5 to control
the orbit of two bunches independently.
 Large aperture pulse Q will be installed at the end point of J-arc to opticallymatch the e- and e+ (primary high charge e-) beam independently.
 Install 4 sets of large aperture pulse Q magnets around 1-2 sectors to make
optical matching for both high energy e- and low-energy e+ beams and to
reduce the emittance growth consequently.

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Design study for LS2 upgrade We are revisiting the investigation of IR (including QCS) section
to achieve much higher luminosity.
 Beam optics study with possible QCS design.
 QCS group are starting considering magnet re-design.
 Vacuum group and QCS group are working for the possible
mechanical design.

24Summary
 Peak luminosity of 4.7x1034cm-2s-1 has been achieved Demonstrated stable operation over 1A in the LER (withsmaller bunch current less than 0.7mA/bunch) Sudden beam loss is serious challenge to increase luminosityand beam current, up to now.
 Many other challenges:• Beam blowup in LER• Beam line deformation with HER beam current• Shorter beam lifetime; both dynamic aperture and physical aperture (beamcollimators), need to clarify the effect of crab waist.
• Injection efficiency, long-term stability of the injector.

 Several upgrade items during long shutdown 1.
 Re-starting design study for LS2 upgrade.
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